1. Field of the Invention
This invention relates to an image forming apparatus including separation means for separating a recording material conveyed by a recording-material conveying sheet from the recording-material conveying sheet. For example, the invention relates to an electrophotographic-type or electrostatic-recording-type image forming apparatus.
2. Description of the Related Art
One type of known image forming apparatus includes a transfer device for transferring a toner image formed on an image bearing member onto a recording material. The recording material is carried on a recording-material carrying member, such as a transfer sheet or the like, and is adhered to the carrying member by electrostatic attraction generated by applying an electric field to the recording material.
The transfer device includes a separation pawl for separating the recording material, on which the image has been transferred, from the transfer sheet, and a pushing-up roller for deforming the transfer sheet by applying upward pressure from the inside toward the separation pawl so that the separation of the recording material by the separation pawl is assuredly performed.
The separation of the recording material from the tranfer sheet using the separation pawl and the pushing-up roller is disclosed in U.S. Pat. No. 5,130,758.
An examplary pushing-up roller 41 is shown in FIGS. 11 and 12. Roller 41 is substantially cylindrical. A chamfer 41b is formed at each of the roller's edge portions 41a.
A pushing-up roller 51 shown in FIGS. 13 and 14 includes a substantially disc-shaped main body 51a and a rotation shaft 52 mounted on the center of the main body 51a.
However, the transfer devices including the pushing-up rollers shown in FIGS. 11 through 14 have the following problems.
For example, when the pushing-up roller 41 shown in FIGS. 11 and 12 is used, the transfer sheet is not locally deformed because the roller 41 is elongated and tends to deform the transfer sheet by uniformly pushing up a relatively wide portion of the transfer sheet. As a result, the leading edge of the recording material is in some cases not separated from the transfer sheet, thereby causing a sheet jam.
When the pushing-up roller 51 shown in FIGS. 13 and 14 is used, the transfer sheet will be locally deformed because the roller 51 has a width acting on the transfer sheet that is much smaller than in the case of the pushing-up roller 41, so that separability of the recording material from the transfer sheet is sufficient. However, since strong shock and distortion act on only a limited portion of the transfer sheet, the transfer sheet tends to permanently deform or break after continuous use. In particular, large pressure is applied to the portions of the transfer sheet in contact with the edge portions of the pushing-up roller 41, so that these portions of the transfer sheet tend to be permanently deformed or broken.
Once the transfer sheet has been permanently deformed, the close contact between the recording material and the transfer sheet is degraded, resulting in the toner image being insufficiently transferred and the transferred image being greatly disturbed.
When the transfer sheet has broken, the operation of the apparatus is disordered. Particularly when, for example, electric-field transfer is used in the transfer process, a concentrated electric field is generated at the broken portion, thereby degrading the photosensitive drum carrying a latent image.